1. Field of the Invention
The application generally relates to a method for measuring the concentration of a gas component in a measuring gas by a gas analyzer. An absorption line of the gas component is varied as a function of the wavelength of the light of a wavelength-tunable light source within a periodically sequential scanning interval. The absorption line of the gas component is modulated with a frequency (f0). The modulated light is guided through the measuring gas onto a detector. A measurement signal generated by the detector is demodulated upon determining a harmonic (nf0) of the frequency (f0). A measurement result is produced by fitting a setpoint curve to the profile of the demodulated measurement signal. Both the demodulated measurement signal and the setpoint cure are filtered with the aid of the same filter function. The filter function is operative to suppress noise signal components of the demodulated measurement signal that disturb both signal components of the demodulated measurement signal and the setpoint curve.
2. Related Art
In EP 1 475 618 B1, a wavelength-tunable light source in the form of a laser diode generates light in the infrared region, which is led through a process gas (measuring gas) to be measured and subsequently detected. The wavelength of the light is tuned to a specific absorption line of the gas components respectively to be measured, the absorption line being scanned periodically as a function of the wavelength. To that end, the laser diode is driven with a ramp-shaped or triangular current signal within periodically sequential scanning intervals. During the comparatively slow scanning of the absorption line, the wavelength of the generated light is additionally modulated sinusoidally with high frequency and small amplitude. Since the profile of the absorption line is not linear, harmonics above the modulation frequency are also generated in the measurement signal obtained by the detector. The measurement signal is usually a modulated nth harmonic, preferably the second harmonic, demodulated by a phase-sensitive lock-in technique, and evaluated for each scanning interval to yield a measurement result. In small modulation amplitudes, the detection of the nth harmonic is directly proportional to the nth derivative of the direct measurement signal. The evaluation is performed, for example, by fitting (e.g., curve fitting) of the profile, to be expected in an ideal case, of the demodulated measurement signal (setpoint curve) to the actual profile (actual curve). Finally, the concentration of the gas component to be measured is determined from the measurement result obtained in this case.
Temperature changes within the gas analyzer can lead to changes in the measurement results. This characteristic, referred to as drift, of the gas analyzer greatly limits its measurement response and the applications to be implemented. One cause of the drift can be, inter alia, etalons in the optical beam path. The etalons lead to periodic structures in the profile of the demodulated measurement signal, wherein the structures lie in the frequency range of the absorption signal to be expected. During curve fitting, this leads to badly fitted functions and deviations of the determined concentrations from the actual concentrations of the gas component to be measured.
In order to suppress the noise signal components, it is known from the abovementioned EP 1 475 618 B1 to guide a portion of the light generated by the light source directly to a monitor detector, and to demodulate the monitor signal obtained at the nth harmonic, and evaluate it. Each deviation of the demodulated monitor signal from a zero line is based on an optical disturbance that, to the extent that it lies in the region of the light source or in the section of the beam path used in common by the measurement channel and monitor channel, also impairs the measurement signal. The disturbance is compensated by a predistortion of the driving of the light source when the wavelength of the light is additionally modulated with the nth harmonic, the modulation intensity being a function of the demodulated monitor signal.
The decoupling of a portion of the generated light on the monitor detector is, however, linked to increasing complexity of design and circuitry, which may lead to a high disturbance susceptibility. Moreover, it is not possible to compensate for disturbances of the measurement signal occurring outside the common sections of the measurement channel and monitor channel.
It is known from EP 2 336 738 A1 or EP 1 927 831 A1 to vary the optical wavelengths, for example, by mechanical vibration of the light source, and to average out the interfering periodic structures from the demodulated measurement signal. However, it is possible thereby to reduce only specific interference disturbances generated by parallel optical surfaces in the beam path.
An object of the present disclosure is reducing changes in the measurement results resulting from disturbing influences, such as, for example, temperature changes in the gas analyzer.
In accordance with an embodiment of the present disclosure, the object is achieved when both the demodulated measurement signal and the setpoint curve are filtered with the aid of the same filter function, the filter function being operative to suppress noise signal components of the demodulated measurement signal that disturb useful signal components of the demodulated measurement signals and with the setpoint curve.
If the disturbances (e.g., such as the abovementioned etalon disturbances) occur chiefly at specific frequencies, it is thus possible for them to be damped by filtering or to be removed from the demodulated measurement signal. If the spectra of the noise signal and useful signal components of the measurement signal overlap one another, the filter also influences the useful signal; put differently, the filtered useful signal changes shape due to the filtering.
Accordingly, the setpoint curve, to which fitting is to be performed, is filtered by the same filter function as the demodulated measurement signal. The filtered setpoint curve is fitted to the filtered demodulated measurement signal. An object of the filtering is to dampen the noise signal components more strongly than the useful signal components, and to achieve an improvement in the useful/noise signal ratio. This is possible in principle when the spectra of the noise signal and useful signal components are not the same. Thus, the noise signal component of an etalon is wider than the useful signal component, and therefore includes more low-frequency components than the useful signal.
According to the invention, the filter function is designed to suppress the noise signal components of the demodulated measurement signal that disturb the useful signal components of the demodulated measurement signal and with the setpoint curve. In as much as the demodulated measurement signal has previously already been filtered, the filtering has, however, been performed in a truly broadband fashion, because the spectrum changes with the width of the absorption line that, in turn, depends on the pressure. The bandwidth of the previous filtering has therefore been selected such that neither narrow nor wide demodulated measurement signals were disturbed. However, the curve fitting algorithm is scarcely disturbed by frequencies not included in the signal to be fitted, and therefore even has a good filtering effect. Thus, in contrast with the disclosed filtering, an exact fitting of the filter or the filter function to the desired function (e.g., matched filter) would lead only to slight improvements, in which case the disturbing frequencies would continue to be effective because they lie in the same region as the useful signal.
Since the disturbances occur chiefly in the low frequency region, it is preferred to use the filter function of a highpass filter. The disadvantage of the method is a lower signal-to-noise ratio at a constant ambient temperature, since signal components of the useful signal are filtered out. This reduces the temperature dependence at the expense of the limit of detection at a constant ambient temperature. However, since the temperature dependence is much larger than the noise at a constant ambient temperature, it is possible to neglect this disadvantage. Apart from that, noise can be suppressed relatively easily by averaging or Kalman filtering, whereas etalon disturbances can be removed subsequently only with difficulty.